Some isotopes of lanthanide metals are used as neutron poison or neutron absorber in nuclear reactors. This is the case in particular with gadolinium (Gd) and erbium (Er). Not all gadolinium isotopes (152, 154, 155, 156, 157, 158, 160) and erbium isotopes (162, 164, 166, 167, 168, 170) are equally advantageous and manufacturers are looking to isolate the most favorable isotopes. The 155 and 157 isotopes of gadolinium and the 167 isotope of erbium are the isotopes having the best neutron absorption capacities and are the isotopes of choice as products which absorb neutrons in fuel elements for nuclear power stations.
For further information on the isotopes of lanthanide metals, see Handbook of Chemistry and Physics, 73rd edition, 1992-1993.
The separation of the isotopes of the same element is one of the most difficult technical problems to solve, whatever the scale of separation chosen. It increases in difficulty as the difference in relative mass between the isotopes decreases, e.g. between 1 and 2% for lanthanide metals.
The separation of the isotopes of elements such as Ca or Na in the liquid phase (aqueous or organic phase) using complexing agents was carried out for the first time by the teams of Jepson and DeWitt (J. Inorg. Nucl. Chem., 38, 1175, 1976) and of Heumann and Schiefer (Z. Naturforsch., 36b, 566, 1981). The techniques employed involved liquid-liquid extraction and ion-exchange resins using specific complexing agents, such as crown ethers (dicyclohexano[18]crown-6) or cryptands (cryptand[2.2.2]).
The separation of the isotopes of rare earth metals by an ion-exchange resin (chromatography) in the liquid phase involving ion-exchange resins and an eluting solution comprising a ligand for the isotopes has also been provided. Thus, European Published Patent-A-173 484 provides such a technique for the separation of gadolinium isotopes using from 5 to 30, preferably from 20 to 30, columns comprising either an anion-exchange resin or a cation-exchange resin as stationary phase. In the first case, the eluant comprises ammonium nitrate in aqueous methanol and, in the second case, EDTA. Mention may similarly be made of J. Chen et al., Journal of Nuclear Science and Technology, 1992, 29 (11), 1086-1092, and I. M. Ismail et al., J. Chromato. A, 1998, 808, 185-191.
For reasons related essentially to the difficulties in controlling the elution of the isotope peaks, Published Patent WO-A-96/00124 has attempted to improve the separation of Gd isotopes on an ion-exchange resin. The method disclosed then requires a mobile phase preferably formed of an aqueous acid, preferably nitric acid. A similar method is provided by Published Patent WO-A-96/00123 for the separation of the erbium isotopes.
Other authors have provided a redox system by chemical exchange in liquid-liquid extraction systems with ligands of HDEHP or TBP type, for the separation of europium and cerium isotopes (W. Dembinski and T. Mioduski, Journal of Radioanalytical and Nuclear Chemistry, Letters, 1995, 199(2), 159-171; W. Dembinski et al., Journal of Radioanalytical and Nuclear Chemistry, Articles, 1991, 149(1), 169-176).
French Published Patent 2 214 509 provides for the separation of the 44 and 40 isotopes of calcium by a liquid-liquid extraction process based on the use of crown ethers and of solvent of water-alcohol type or of chlorinated solvent.
The stakes are therefore high in the separation of isotopes. However, the various techniques have drawbacks, e.g. the complexity of the arrangements to be employed; the cost and the scale of the plants; the energy expenditure; the production of liquid or solid byproducts to a greater or lesser extent toxic; the use of solvents presents the problem of their separation and of their reprocessing, for the purpose of recycling them and of protecting the environment.